The 18q- syndrome is the most common terminal deletion disorder in humans. Previous G-banding studies suggest that the loss of a critical band, 18q2 1.3, results in mental retardation, craniofacial anomalies and metabolic defects. However, it is difficult to reconcile the consistent loss of a single region with the large variability in clinical phenotype. Therefore, the investigators developed a genome-based approach to assess the extent chromosomal loss in this disorder. First, they constructed a dense physical map and a 10-15 Mb size yea artificial chromosome (YAC) contig that spanned the putative critical region. The map, YACs and derived probes provided the foundation for subsequent analyses. Next, they assessed the extent of chromosomal loss in a cohort of 18q- syndrome patients by using fluorescence-activated chromosome sorting, PCR and FISH. Bivariate flow karyotypes revealed heterogeneity among the deletions, which ranged in size from 9 to 26 Mb. To confirm this heterogeneity at a molecular level, deleted and normal chromosornes 18 were collected by flow-sorting, pre-amplified by random priming and assayed for marker content by the PCR. This revealed a successive set of deletion breakpoints within 18q21.2-q22.2. These results suggest variability in clinical phenotype may be more representative of a contiguous gene syndrome with a deficit of 18q22.2-qter, rather than the loss of a single critical region within 18q21.3. A more genotype- phenotype correlation awaits the construction of an informative transcriptional (gene) map of the region. Finally, they gained initial insights into how a de1(18) might arise by cloning and sequencing the "deletion" breakpoint from an 18q- syndrome patient. This analysis showed that 1) the breakpoint occurred within a newly discovered cluster of four closely related serine proteinase inhibitor (serpin) genes and 2) novel satellite III DNA sequences, not ordinarily found on chromosome 18, were appended to the truncated chromosome. This suggested that the "de1(18)" was actually a cryptic subtelomeric translocation or the consequence of some other type of illegitimate recombination event. Subsequent analyses of additional 1 8q. patients suggest that this type of recombination event may not be an isolated occurrence. The current proposal builds upon their initial observations and tests the hypothesis that cloning S sequencing the breakpoint regions will help identify new genes associated with the clinical phenotype provide further insights into the mechanisms of both terminal DNA loss and chromosomal stabilization in congenital terminal deletion disorders. The specific aims are to: 1) Decipher the relationships between chromosomal loss and clinical phenotype by identifying and/or mapping genes and functional domains in 18q21. 2) Determine whether particular DNA motifs on 18q predispose to chromosomal loss, an 3) Determine the nature and origin of the telomeric flanking-elements that help stabilize the del(18)s. An increased understanding of how terminal deletions arise provides important insights into the etiology of this major cause of birth defects and mental retardation.